Stents are commonly used as a means of supporting weakened sections of blood vessels, glands or ducts, to re-open lumens of collapsed or occluded vessels, glands or ducts, or as means for treating aneurysms and other diseases. Stents have also been useful as a means of delivering therapeutic agents to specific sites within the vascular or ductal structures of organs. Stents may also be manufactured to be bioabsorbable such that the body absorbs them over a period of time. However, each of the features creates challenges in the design and manufacture of stents
For example, drug-containing stents are constructed of polymeric fibers compounded with the drug of interest. The mixture is then formed into structural elements of a stent by processes such as co-compression molding, co-injection molding, or co-extrusion. For example, in a conventional co-extrusion process, one or more therapeutic agents are compounded with a polymer resin. The compounded resin is then melt extruded to form elongated fibers from which the stent can be fabricated. However, these processes suffer from significant limitations. First, the temperature at which the polymer can be melted may cause degradation of the therapeutic agent.
Similarly, the ratio of drug to polymer may have a significant impact on the strength and/or flexibility of the stent. To overcome this, it is common to maintain the drug-polymer ratio low in order to avoid compromising strength. However, this limits the amount of bioavailable drug that can be packaged within the stent, and which would be available in vivo for therapeutic use.
Other ways in which to provide sufficient structural strength to a stent can involve the use of high or ultra-high molecular weight fibers. While these materials are effective to provide the desired structural strength to the stent, they suffer from the drawback that they have very long degradation time. This can lead to issues such as foreign body reactions.
It is also difficult to vary the composition of the stent over its length in order to provide variable strength and/or flexibility in different regions of a stent. For example, in order to make a region more flexible normally requires making the stent thinner, while increasing strength typically requires the stent framework to be thicker. As a result, strength and flexibility are generally opposing outcomes of stent design.
An additional advantage provided by stents is that they allow for treatment of medical conditions using minimally invasive transcatheter delivery methods. To make use of minimally invasive transcatheter delivery methods typically requires that the stent material be visualizable using medical imaging techniques such as fluoroscopy. In order to do this, the stent must include a radio-opaque material that can be visually distinguished from surrounding anatomical structures.
Like is done with therapeutic agents incorporated into stents, frequently the radio-opaque material is co-extruded with the polymeric fiber. However, this technique alters the mechanical properties of the stent, as well as reduces the proportion of the stent that is available for inclusion of the desired therapeutic agent(s).
Thus, there is a need not met by the prior art for a polymeric stent that is generally bioabsorbable, which can be variably flexible over the length of the stent, which provides openings to branching vessels, and which can be visualized by conventional medical imaging techniques both during and after placement into a patient.
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Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.